The current macrostability safety assessment for primary river dike trajectories in the Netherlands is applied to approach the failure probability of a dike during high water events. However, in the current schematization process that is described in the Wettelijk Beoordelings Instrumentarium (WBI) to assess the macrostability, aleatory and epistemic uncertainties are approached ’sufficiently safe’ by applying design values based on expert judgement via a semi-probabilistic assessment. Several primary river dike sections in the Alblasserwaard do not suffice the current safety standard set for the failure mechanism macrostability. The region is composed of a highly complex subsurface with large spatial variation, resulting in large schematization uncertainties for the macrostability assessment of the primary dike trajectories of the Alblasserwaard. With the recent development of full-probabilistic analysis possibilities in software such as D-Stability, it becomes possible to consider uncertainties as a stochastic variable in the macrostability safety assessment. Including schematization uncertainties within the macrostability safety assessment will improve the approximation of the failure probability of the primary dike trajectory. The largest schematization uncertainties in the macrostability safety assessment are currently considered to be the schematization of the subsurface in a vertical soil profile and the uncertainties in the schematization process of the pore water pressures in the dike during high water events. These uncertainties will be included in the calculation process to investigate the influence on the expected reliability of the primary dikes in the Alblasserwaard region. The subsurface schematization uncertainties are investigated by using soil scenarios to investigate the influence of local subsurface schematization in the vertical soil profile. The simplification of the soil profile and position of the soil layers is considered. The pore water pressures are separated into three components: the hydraulic head in the aquifer, the intrusion length, and the phreatic line. Each component will be included as a stochastic variable in the stability analysis. Fragility curves can be applied to describe the distribution function for each pore water pressure component, where the combined fragility curve will provide the combined failure probability and reliability index that includes the schematization uncertainty of the pore water pressures considered.
The soil scenarios can be applied to include schematization uncertainties of the subsurface in the macrostability safety assessment. The analysis showed that the simplification of the subsurface schematization only has a minor influence on the reliability index and failure probability of the case study dike cross-section Kortenhoevendijk. The schematization uncertainties of the pore water pressures can be considered in the macrostability safety assessment by combining the fragility curves of each component describing the pore water pressures underneath the dike. Results of the pore water pressure analysis are that failure probability is improved significantly for case study Kortenhoevendijk by a factor 1000 and case study Bergstoep by a factor 10. The approach to consider schematization uncertainties in the macrostability safety assessment via a full-probabilistic analysis can be used for dike sections prone to the uplift mechanism. This approach provides insight into the influence of schematization uncertainties on the failure probability of a dike cross-section. Including the pore water pressure schematization uncertainties in the macrostability safety assessment can have a significant impact on the outcome of the assessment. Including these uncertainties can make the difference between deciding whether a dike trajectory needs reinforcement, or deciding that reinforcement is not necessary.

The Water Authority Rivierenland is responsible for periodically assessing the safety of the dike trajectories in the Alblasserwaard region. In 2012, the safety assessment for inner slope macro-stability was performed based on stress dependent design values for the shear strength, based on the cell test
collection of the Water Authority. The most recent assessment on the inner slope macro-stability of the Lekdijk has shown a significant deviation from the previous assessment. According to these results, large scale reinforcement works are requested. Before starting any additional soil investigation, the water authority is interested in investigating the sensitivity of the schematization of the shear strength. The transition from the cell test collection to the triaxial and direct simple shear test collection, and therefore the transition from using the Mohr-Coulomb calculation model to the SHANSEP formulation, is expected to have the most impact on the outcome of the macro-stability safety assessment. The influence of the two test collections on the macro-stability safety assessment is analyzed by using a D-Geo-Stability model from the previous safety assessment for one cross-section of the Lekdijk, and transferring the model to D-Stability. The D-Stability model with the cell test collection parameters and the Mohr-Coulomb shear strength calculation model can be adjusted to the triaxial and direct simple shear test collection with SHANSEP shear strength calculation model. The transition from drained to undrained modelling results in a decrease of the shear strength of the soil around the failure surface. Therefore, the transition from the previous to the new test collection has resulted in a lower safety factor in the macro-stability analysis, having a negative impact on the overall macro-stability safety assessment.